R129 - model code for Mercedes-Benz SL-Class
Two door roadster produced from 1989 to 2001
A total of 204,940 units were produced. Two-door convertible with a soft and removable hard top, rear-wheel drive, and engines ranging from 2.8 to 7.3 liters represented the pinnacle of technological advancement at the time.This classic car, which made a lasting mark on the automotive industry in the early 1990s and continues to excite modern car enthusiasts, combines incredible performance, cutting-edge technology and a beautiful appearance. With the purpose of taking you on a tour of the Mercedes SL-Class (R129) and highlighting its noteworthy features, background, and reasons for being considered a timeless classic, we republish this comprehensive guide ...
R129 is in between R107 and R230 SL-Class models
The 1984 design of the R129 was based on the modified floorpan of the Mercedes-Benz W124. The new SL-class became available for sale in March 1989 during the Geneva Motor Show, with left-hand drive models hitting the market in the summer and right-hand drive versions following in the fall. In the European Car of the Year competition, it secured the second position, with the Citroen XM taking the top spot.Tradition and Innovation
The convertibles from Mercedes-Benz have always set the standards for luxury sports cars. With the R129 SL on 89, not only has this tradition has been continued - a car has been created which is quite unique in the sum total of its features and qualities. On the one hand, the convertible has a dynamic appeal comparable to that of the legendary 300 SL from the fifties; on the other hand, it incorporates an abundance of innovative engineering features allowing the previously conflicting criteria of sporty handling and ride comfort to be reconciled without compromise. The R129 Mercedes-Benz convertibles, the 500 SL, 300 SL-24 and 300 SL with eight and six-cylinder engines developed 240 kW/326 hp, 170 kW/231 hp and 140 kW/190 hp respectively, offered a hitherto unequalled fascination of open driving in typical convertible style.Comfort: electro-hydraulically operated soft top
To make open driving both pleasurable and comfortable, the convertible was fitted with an electro-hydraulically operated soft top, in addition to the standard hard top. At the push of a button, the soft top was opened and folded away under its cover or closed and locked within 30 seconds. During this process, the side windows were retracted and subsequently return to their initial position. To ensure that even the open car meat Mercedes-Benz's exacting safety standards, a completely new, extendable roll-over bar was developed at that time. When retracted, it is flushed with the soft top cover. When an electronic control unit registered a critical situation which might lead to a roll-over, the bar moved up within 0.3 seconds. It could also be extended at the push of a button. Another new feature for pleasurable open driving even at high speeds was the so-called draught-stop, a framed net attached to the roll-over bar which effectively reduced unpleasant draught inside the car. It also reduced wind noise to a minimum and in conjunction with the generously dimensioned heating system for the interior, it gone a long way towards significantly extending the "open-air season".Sports-car performance
The performance was in keeping with the convertible's claim to sports-car characteristics. Depending on the model and transmission version, the new SL accelerated from standstill to 100 km/h in between 6.2 and 9.3 seconds, its top speeds ranged from 223 to 250 km/h.The two six-cylinder models was fitted as standard with a five-speed gearbox, the 300 SL-24 was also available with a sporty gearbox version. A four-speed automatic transmission was optionally available for the two six-cylinder models and standard on the 500 SL because of the latter's exceptional power potential. A new feature on the automatic transmission was a special parking lock which ensured that the ignition key can only be removed when the parking lock has been engaged. Conversely, the parking lock could only be disengaged with the ignition key in drive position.
Four-valve engines on 1989
There is a choice of three engines for the new SL:- 500 SL: Eight-cylinder four-valve five-litre engine, 240 kW/326 hp
- 300 SL-24: Six-cylinder four-valve three-litre engine, 170 kW/231 hp
- 300 SL: Six-cylinder three-litre engine, 140 kW/190 hp
Refined emission control system
Like all other Mercedes-Benz petrol-engined cars, the SL models were fitted as standard with the closed-loop three-way catalytic converter. It was a completely newly developed double-fluted catalytic converter with two ceramic monoliths fitted side by side in one housing. Its advantages were high long-term stability, reduced exhaust gas backpressure and low noise emissions from the exhaust system. The evaporation gases from the fuel tank were trapped by an activated charcoal filter and mixed with the combustion gases at specified intervals. Over and beyond that, a number of features have been incorporated which reduce pollutant emission as early as during the combustion process. Among these is an air pump which during the warm-up phase channels fresh air through the cylinder head thus provided for the afterburning of unburnt hydrocarbons (HC) and carbon monoxide (CO) directly at the exhaust valves. An important contribution towards improved emission quality is rendered by the new mechanical/electronic CIS-E-V injection system. Among other things, it caused the oxygen sensor to be heated, monitors exhaust gas recirculation as well as secondary air injection, and controls the venting of the activated charcoal container.R129 seats, more than just a car seat
With the SL, Mercedes-Benz introduced a completely new type of seat. This integral seat incorporated many features which not only enhance comfort but also increased passive safety. The following functions are integrated in the electrically adjustable seat:- three-point inertia-reel seat belt;
- belt tensioner;
- seat belt sash guide height adjustment;
- head restraint height adjustment.
The adjustment of head restraint and sash guide height was coordinated. This ensured that the belts fit optimally, thus increasing occupant protection. The belt tensioner, which in the event of an accident pulled the belt tight within fractions of a second, was a standard fitting.
All these features combined into a comprehensive passive safety system, ranging from the front end structure specially designed to gave maximum protection in an offset frontal collision to the new, automatically extended roll-over bar system which needed not fear comparison with the saloons.
Sporty handling and ride comfort as required
The most important contribution to active safety was rendered by good handling and ride characteristics. The SL was fitted with the shock absorber strut independent front suspension and the multi-link independent rear suspension, a combination which had already proved its outstanding qualities in other Mercedes-Benz car series. In addition, all SL models including for the first time the six-cylinder models was optionally available with acceleration skid control (ASR). In combination with the standard ABS and electronic engine control, this ensured optimum traction and maximum safety reserves even under difficult road surface conditions.A completely new feature on the convertible was the adaptive damping system (ADS) which was combined with automatic level adjustment and control. On the move, ADS permanently adjusts the damper characteristics to the prevailing driving and road conditions. The system changed from one damper setting to another there was altogether four fully automatically and imperceptibly. With this innovative technology on 89, the previously conflicting targets of ride comfort and sporty handling have been reconciled. The electronics always and accurately provided for the suspension characteristics which was most appropriate for the motoring style, driving situation or road condition at any given point in time.
Ultra-modern production facilities in Bremen on 89
For the first time, a Mercedes-Benz SL was manufactured in Bremen. In conjunction with Mercedes-Benz's knowhow in sports car manufacture, highly modern production facilities ensured that the most exacting quality standards was met for the convertible. The quality of the new SL could only be achieved by means of the use of the most advanced production technology when it comes to torsional rigidity, dimensional accuracy, the passive safety of the car's structure and all other aspects relevant to quality. Up until then, this degree of scrupulousness has not been conceivable in view of the production volume of the luxury sports car. It was planned to produce some 2,000 convertibles per year in Bremen once production run in full swing. Being an export harbour and thus a link with all countries around the world, the former hanseatic city of Bremen was a particularly good location in which to proceed the international SL myth.Technical information
Body
Integral all-steel body, open, with detachable hardtop.Chassis
Front axle: Shock absorber strut independent front suspen- sion with anti-dive control, coil springs on wishbones, gas-filled shock absorbers, negative steering offset, torsion bar stabilizer.Rear axle: Multi-link independent rear suspension with anti-squat and anti-lift controls, coil springs, gas-filled shock absorbers, torsion bar stabilizer, Level control all round, and electronically controlled damping system (ADS) available as optional extra.
Brake system: Hydraulic two-circuit brake system with vacuum brake booster, 4-piston fixed caliper disc brakes, ventilated, front 2-piston fixed caliper disc brakes, rear anti-lock braking system (ABS).
Engine
Performance - choice of three powerful engines
In engine technology, Mercedes-Benz set new standards with a refined emission control system and automatic camshaft adjustment, the latter improving the new four-valve engine's smooth running characteristics and raising its torque. The new SL was available with either of three high-capacity and smooth-running engines which were quite different in their performance characteristics:- Three-litre six-cylinder two-valve in-line engine, 140 kW / 190 hp
- Three-litre six-cylinder four-valve in-line engine, 170 kW / 231 hp
- Five-litre four-valve V8 engine, 240 kW / 326 hp
Modified three-litre six-cylinder two-valve engine
The three-litre six-cylinder two-valve engine was largely identical with the well-known power unit which is also installed in the Mercedes-Benz mid-series and S-class cars. A new feature of this engine is the "open" combustion chamber. To ensure more efficient combustion of the fuel/air mixture, there was no longer a quench zone in the cylinder head. This intensifies combustion and thus lowers the raw emission of unburnt hydrocarbons (HC).Engines with catalytic converter were fitted with an air pump which channels additional fresh air through the cylinder head and injects it into the exhaust ports where it is mixed with hot exhaust gases. The afterburning process of the exhaust gases, which starts immediately after the engine was started, accounts for lower pollutant emission even at this stage. Over and above this, this process improves the conditions for subsequent catalytic emission control. NOX emissions are reduced by means of exhaust gas recirculation, a standard feature.
All detail improvements to the cylinder head, a modified camshaft and the double-fluted catalytic converter account for improved torque and engine output ratings and even smoother running characteristics.
New three-litre six-cylinder four-valve engine
The three-litre six-cylinder four-valve engine shares a large number of components with the six-cylinder two-valve engine. A completely new development is its four-valve cylinder head with inlet camshaft adjustment.The valves, fitted at an angle of 50 and arranged in compact, pent-roof shape combustion chambers with level pistons and centrally positioned spark plugs, are actuated by one inlet camshaft and one outlet camshaft via maintenance-free hydraulic bucket tappets. The two camshafts are driven by a duplex roller chain.
Camshaft adjustment, a new feature used for the first time by Mercedes-Benz, improves the cylinders' volumetric efficiency in dependence of engine speed and torque, resulting in im- proved idling and higher torque. Furthermore, the generously dimensioned intake and exhaust ports, arranged according to the cross-flow principle and converging inside the cylinder head, account for low gas cycle resistances.
The ports are arranged to as to provide for the optimum positioning of the injection nozzle relative to the cylinder for good cold-starting behaviour. The insulated nozzle brackets are air-bathed so as to ensure reliable hot-starting. Furthermore, special intake manifold seals prevent the in- jection valves from being heated up.
To minimise noise emission even at this engine's high specific output, the pistons are sprayed with engine oil from below. The oil reduces piston temperature, thus ensuring unchanging piston clearances. This significantly improves piston lubri- cation and thus reduces noise emission.
As in the six-cylinder two-valve engine, the four-valve unit's exhaust emissions are reduced by means of a closed-loop three-way catalytic converter, additional fresh-air injection into the exhaust ports and exhaust gas recirculation.
The single-part cylinder head has sufficient rigidity for valve train support and combustion chamber stabilisation, in spite of its low weight. Fitted with the proven crank assembly, this engine is an exceptionally dynamic unit with a maximum engine speed of 7000/min. A torque of over 250 Nm is on tap over a broad engine speed band, from 3000 to 6500/min, and the four-valve engine's maximum torque of 272 Nm is reached at 4600/min.
The optimised four-valve engine is fitted with a combined mechanical/electronic fuel injection system which, because of its capacity, is also used for the V8 engine.
New five-litre four-valve V8 engine
The most powerful engine for the new SL is a five-litre V8 unit whose increased power output is predominantly due to its improved gas cycle employing a total of 32 valves.The cylinder crankcase of the new four-valve engine has been derived from the proven M 117 S-class engine. Its high power. output of 240 kW/326 hp has been achieved by means of substantial modifications to the light-alloy unit. In spite of this, it has been possible to keep the V8 crankcase's weight down.
The iron-coated pistons and the cylinder contact surfaces form a low-wear combination. The crankshaft is forged out of high-strength, micro-alloyed tempered steel, with all radii being additionally hardened. Micro-alloyed tempered steel is also used for the connecting rods.
The two cylinder heads, equipped with 32 valves and four camshafts, have been completely newly developed. Two inlet and two outlet camshafts, jointly driven by a duplex roller chain, actuate the valves via maintenance-free hydraulic bucket tappets. The valves are asymmetrically positioned at an angle of 37.7 degrees relative to the cylinder axis. With the steeply angled position of the exhaust valves, the V8 engine could be kept rather small in width.
This large-capacity four-valve engine, too, is fitted with electronically controlled inlet camshaft adjustment, developed by Mercedes-Benz. This improves the cylinders' volumetric efficiency, and thus torque, power output and idling.
Because of the higher oil flow rate in the four-valve cylinder heads, each cylinder is fitted with a separate oil return passage. Two oil splash shields in the oil sump reduce oil foaming, which can damage the hydraulic bucket tappets. compact, pent-roof shape combustion chamber with centrally positioned spark plug has been optimised with a view to flame propagation. Short flame travel and defined turbulences account for optimum energy utilisation, knock resistance and low raw emissions.
The electronic injection and ignition systems, exhaust gas recirculation control and the air injection into the exhaust ports correspond to the six-cylinder four-valve engine's systems in terms of design and function.
Automatic camshaft adjustment
A rigid camshaft with non-adjustable ignition timing will always constitute a compromise with a view to torque on the one hand and peak performance on the other. With inlet camshaft adjustment, now introduced by Mercedes-Benz, individual valve overlap in the four-valve engine is now modified according to the engine's operation condition. Hence, with the new four-valve technology from Mercedes-Benz, compromise is a thing of the past.With automatic inlet camshaft adjustment, there is virtually no valve overlap at idle speed, accounting for smoother running and reduced pollutant emission and fuel consumption.
At medium engine speeds, greater valve overlap at TDC improves the cylinders' volumetric efficiency and raises the torque.
For high output ratings at high engine speeds, the closing of the inlet valves is delayed.
Inlet camshaft adjustment is controlled by the injection system's control unit in dependence of torque and engine speed. The adjustment mechanism proper, however, is fitted in the camshaft gear.
The camshaft gear continues to be fitted on the camshaft and is driven by the crankshaft via a roller chain. A new feature is the ability of the camshaft to turn relative to the camshaft gear.
At the front end of the camshaft with longitudinal toothing, an adjustment piston with helical outer toothing is fitted which meshes with the inner helical toothing of the camshaft gear. Depending on the position of the piston, the position of the camshaft relative to the camshaft gear can thus be altered, thus changing the valve timing by the inlet camshaft.
To change the position of the adjustment piston, a magnet activated by the injection system's electronic unit operates a valve, thus forcing in engine oil which moves the piston into the desired position. The camshaft is simultaneously adjusted via the helical toothing.
KE 5: new injection system
More exacting demands made on the injection and ignition systems' electronic control units call for greater computer capacity. Hence, each system has its own control unit. These units are, however, linked by means of an interface so that data can be exchanged. It has thus become possible to incorporate additional engine management functions. Over and above this, the previous functions can be performed more efficiently and in a more intelligent manner, and finally, networking also provides for the redundancy of the most important sensors.The new KE 5 injection system operates mechanically/electronically. Fuel metering is effected by means of a fuel distributor activated by an electronic control unit.
The additional functions now incorporated in particular reduce pollutant emissions:
- Oxygen sensor heating increases operational reliability and service life.
- An intermittent cold-starting aid provides for fuel enrichment orientated towards requirements.
- The injection system's control unit also monitors and controls exhaust gas recirculation and additional air injection.
- In the event of failure of important sensors, the electronic unit feeds in a replacement value, thus maintaining operational reliability.
- Fuel tank venting: the fuel vapours from the tank are passed through an activated charcoal filter and mixed with the combustion gases at specified intervals. The venting of the activated charcoal filter is controlled by the electronic unit.
- Engine parameter data are recorded, ensuring optimum control for instance when re-starting the engine. Example: Fuel apportionment in accordance with ambient air pressure may vary, depending on whether the car is driven down in a valley or high up in the mountains. When the engine is re-started, the electronic system will "remember" at what altitude the car was parked and apportion the appropriate fuel quantity for smooth starting.
- Combustion noise is monitored by two knock sensors which are capable of identifying individual cylinders. Any tendency to knock is detected at an early stage, and knock is reliably prevented by a reduction of the relevant cylinder's ignition angle.
Exemplary reduction in pollutant emission
A large number of new and additional technical features have been incorporated in the SL engines to reduce pollutant emission as early as the combustion stage.Among these features is a V-belt-driven air pump. During warm-up, it injects fresh air through the cylinder head and into the exhaust ports. This accounts for the afterburning of unburnt hydrocarbons (HC) and carbon monoxide (CO) directly at the exhaust valves. In this way, emissions are reduced, and the catalytic converter heats up more quickly.
All SL engines are equipped with additional exhaust gas recirculation. Some of the exhaust gas is mixed with the combustion air, thus reducing NOx emission.
Since the cylinder head of the six-cylinder two-valve engine no longer has a quench zone, the fuel/air mixture is burnt more thoroughly, resulting in reduced raw emissions of unburnt hydrocarbons (HC).
Inlet camshaft adjustment in the four-valve engines also contributes to a reduction in the raw emission of unburnt hydrocarbons (HC) and nitrogen oxides (NOx).
Cars for the Federal Republic of Germany and other countries with stringent emission control laws are fitted as standard with the closed-loop three-way catalytic converter.
The SL models broad tunnel accommodates a completely newly developed double-fluted catalytic converter with two ceramic monoliths fitted side by side in a single housing. This new design yields the following advantages:
- High engine output thanks to large catalytic converter cross-section.
- High long-term stability and emission quality consistency.
- Low noise emission through optimised mixing areas.
Modified transmissions
The two six-cylinder models are fitted as standard with a five-speed manual gearbox matched to the powerful engines. Top speed is reached in fifth gear. The 300 SL-24 is optionally available with a manual gearbox with ratios which lend themselves to a sporty driving style. The ratio in fifth gear is 1:1, thus fifth gear is not an overdrive. The driver is able to change through the gears quickly since second and third gear and fourth and fifth gear are in the same plane respectively. Comfort is enhanced by the fact that reverse is fully synchronised.In conjunction with the five-speed manual gearbox, a two-mass flywheel is installed for more effective damping of power train vibrations.
The proven four-speed automatic transmission is optionally available for the two six-cylinder models and standard on the 500 SL. The ratios of this transmission, too, have been matched to the higher engine output ratings.
New features on the automatic transmission are a display for the selector lever position and a special parking lock.
This parking lock is an additional safety element. It reminds the driver to engage the parking lock before removing the ignition key. In addition, the parking lock can only be released when the ignition key is inserted in the ignition lock. It is thus not possible to release the parking lock when the ignition key is removed (example: children playing in the car).
The locking is effected by an additional mechanism with a bowden cable which connects the steering lock with the selector lever. It is thus ensured that the ignition key can only be removed after the selector lever has been put into "p" position. Conversely, the selector lever can only be moved into another position when the ignition key has been inserted and turned.
Sporty performance
The engines and transmission ratios have been designed for sporty performance in keeping with the new SL models' characteristics. Depending on model and transmission version, acceleration from 0 to 100 km/h ranges between 6.2 and 9.3 seconds, and top speeds are between 223 and 250 km/h. Because of its engine power, the 500 SL would be capable of effort- lessly reaching top speeds above 250 km/h yet various elec- tronic features have been incorporated to limit the top speed in the interest of safety, for instance with a view to tyre capacity.The performance of all models has been markedly improved compared with the previous models the new 300 SL-24, for instance, outstrips the previous 420 SL. Fuel consumption is on the same scale as that of the predecessor models; mean fuel consumption based on DIN 70 030 (Euro-Mix) is between 11.0 and 13.0 litres/100 km.
Comfort
SL - a synonym for the most exacting demands on comfort
An abundance of comfort-enhancing features has been incorporated in the new SL series, among them the electro-hydraulically operated soft top, the front seats with integrated seat belts, a newly designed dust filter which cleans fresh air before it enters the interior, and an additional heating system which operates when the engine is switched off. Another new feature is a screen which wards off unpleasant draught in open driving. Over and above this, comfort is, of course, also determined by the engine's smooth running, the noise of air flow around the bodywork and the car's vibration and suspension behaviour. Intelligent solutions had to be found to the problem of meeting the self-imposed exacting demands on comfort in an open car. These solutions have been found by the Mercedes-Benz engineers.Low-vibration bodywork
It requires particularly complex and difficult development work to give an open car pleasant and comfortable vibration behaviour. Extensive measures have to be taken on the bodywork to compensate for the integrated roof. In the design of the new SL, Mercedes-Benz set itself the task of creating a convertible with particularly low vibrations and high torsional rigidity. In extensive tests of the completely newly developed bodywork, it was determined where, how and what additional reinforcements were required to stabilise the open bodywork. These investigations led to the use of additional struts which ensure a higher overall rigidity of the bodywork.In the front section of the car, two profiled struts, fitted with their own vibration dampers, link the front axle carrier with the door sills. In the rear, two tubular struts link the door sills with the spare wheel recess. As a consequence, bodywork distortion of the new SL is as low as that of the Mercedes saloons and coupés which means that the torsional rigidity of the SL bodywork has been noticeably increased, giving exceptionally good vibration behaviour for a car of this type.
Soft top and light-weight hard top
The standard specifications of the new SL include an electro-hydraulically operated soft top and a removable hard top. Both versions can be operated and handled effortlessly.At the push of a button, it takes thirty seconds for the soft top to be folded away or to be extended and locked. At the same time, the side windows and the roll-over bar are lowered and subsequently return to their original positions. Sophisticated technology is required to achieve this ease of operation. The energy required to open and close the soft top is supplied by an electrically driven hydraulic pump which is accommodated, together with the oil reservoir, in the spare wheel recess. The opening and closing is controlled by a micro-processor and monitored by 17 proximity switches; the hydraulic system employs 15 pressure cylinders and 11 solenoid valves.
It goes without saying that a squeeze-in protection system is incorporated just as well as manual emergency control. In "off" position, the hydraulic system is depressurised, and all the relevant points are mechanically locked.
The soft top can be extended and retracted only when the car is stationary and the ignition switched on. The soft-top frame components keep the roof firmly under control. A specially shaped strut serves as a folding aid and prevents the soft-top fabric from lifting off the frame ("ballooning"). Several steel ropes which tension the closed soft top also hold the fabric taut even at high speeds.
The interior roof lining can be replaced easily and is specially coated on the inside; together with meticulously designed roof frame sealing, this goes a long way towards keeping noise levels down.
The noise of air flow around the car, which is generally somewhat louder in a convertible, has been reduced to a minimum by means of careful coordination of form and function in the wind tunnel.
Open driving without draught
Open driving at high speeds inevitably causes draught which feels unpleasant around the head and neck, and which ruffles the occupants' hair and clothes. To make driving at higher speeds nonetheless enjoyable and to increase safety, Mercedes-Benz offers an optionally available draught-stop which does not detract from the pleasure of driving an open This draught-stop is a framed screen which can easily and quickly be fastened to the roll-over bar. With the draught-stop in upright position, draught and wind noises are minimised, and the interior can be effectively heated even during the colder season, so that driving the new SL with the soft top down is possible much more frequently.A hard top weighing only 34 kg
The standard hard top is made of light alloy and weighs only 34 kg 10 kg less than the predecessor's roof. It can be removed and re-installed easily and effortlessly. The roof is locked at four points, and in addition safely located at two points. The roof is locked electro-magnetically via a central switch. The inside of the hard top has a self-supporting sandwich-type plastic lining which absorbs noises and provides efficient insulation against heat and cold.Fresh-air filter
The most noteworthy new feature of the efficient ventilation system is a so-called Elektrete filter which effectively cleans the ambient air drawn in by the blower. The filtering is effected by matting whose polycarbonate fibres are electrostatically charged during the spinning process; the filter completely captures pollen and dust particles as well as liquid particles down to a size of 5 micrometres. It also traps up to 60% of particles which are ten times smaller, for instance bacteria, industrial dust, soot and smog. Depending on the degree of soiling, the filter must be replaced during servicing or at least once a year. Since the air channelling and blower output have been improved, the air flow rate is higher than in the predecessor model, in spite of the air filter. Automatic climate control operates in different modes, depending on whether the roof is opened or closed. This ensures that the heating is controlled largely automatically even with the soft top down.Interior heating even with the engine switched off
To keep the interior comfortably warm even when the engine is switched off, the residual heat of the engine is used to heat the interior, for instance at level crossings, at traffic lights, in traffic jams or during a quick stop off at the shops. By pressing the "REST" key, an electric circulation pump is activated which channels the hot engine coolant through the heating system's heat exchanger. Air quantity and distribution are automatically controlled. The circulation pump switches off after thirty minutes at the latest so that the engine can be re-started reliably.Standard comfort-enhancing features
Many of the comfort-enhancing features are included in the standard specifications, for instance electric windows, central locking and the electrically adjustable steering column in the 300 SL-24 and 500 SL.In addition, other features are optionally available. Among these is a memory feature for electric seat, mirror and steering column adjustment. Once the driver has adjusted the seat, the mirrors and the steering column to the ergonomically optimum positions, he only has to press one of the memory keys to record this setting. After a change in position, the programmed setting for seat, mirrors and steering column can be recalled at the push of a key.
The standard central locking system has been refined by the addition of an "internal locking" feature. This locks all the lockable compartments inside the car - from the spectacles box to the luggage space. A total of six compartments can be centrally locked and thus secured against theft even when the car is open.
Operational comfort is further enhanced by the optionally available infrared remote control unit for the central locking system. The doors can be locked from a distance of up to 6 metres by pressing a transmitter key. For safety reasons, every SL with infrared transmitter is given a different basic signal. In addition, every time the keys are pressed, a different code, ruled by a special algorithm, is transmitted. This virtually prevents other transmitters from taking effect. Open side windows can be closed both by means of the infrared transmitter and by turning the door locking key.
Active safety
Suspension safety plus comfort
The new Mercedes-Benz convertible is a sporty car which at the same time features a high standard of comfort. In suspension design, sophisticated solutions have been achieved, tailored to the characteristics of an open two-seater and meeting the most exacting demands on handling and ride.The most significant contribution towards active safety is rendered by good handling and ride characteristics. This requires first and foremost excellent suspension with precise yet shock-absorbing wheel location elements. For the new SL series, therefore, the proven combination of shock absorber strut independent front suspension and multi-link independent rear suspension has been retained. A new feature, meeting even more exacting demands, is the adaptive damping system (ADS). The proven acceleration skid control (ASR) and automatic locking differential (ASD) are also available for the new SL. These electronically controlled and automatically operating systems relieve the driver from stress and thus improve handling and physiological safety as well as the ease of control.
A car's road-holding stability is predominantly accounted for by its rear axle. This is not least the reason why Mercedes-Benz put so much engineering effort into rear axle design. The multi-link independent rear suspension, tried and tested in the compact and mid-series models, continues to set standards for precise wheel location, thanks to the three-dimensional arrangement of the five links which under spring compression only permit minor, precisely defined kinematic steering effects. In addition, the elasticities (displacements) in the link mounts are tuned so as to prevent toe-in changes at the wheel under longitudinal forces, thus ensuring good straight-line stability. The steering response under lateral forces (cornering for instance) is also precisely defined. In its effectiveness, the SL's multi-link independent rear suspension is comparable to passive rear-wheel steering.
Automatic damping and level control
In modern Mercedes-Benz suspension design, exceptionally high standards of safety and comfort have been reached. In spite of this, the engineers worked on an even further-going, uncompromising solution, the adaptive damping system (ADS) launched for the first time in the SL series. On the move, ADS permanently adapts the damper characteristics to the requirements at any given point in time. The change-over from one damper setting to another there are altogether four, from soft to hard takes place fully automatically and imperceptibly.In the design of a car's suspension system, the engineer is confronted with the problem of reconciling two objectives which due to the laws of physics conflict, namely maximum safety versus optimum comfort. A car can be said to have comfortable suspension if vertical body acceleration on an uneven road surface remains sufficiently limited. To this end, the wheels must be capable of easily evading an obstacle by moving upwards which calls for soft springing and damping. Because of the wheels' inertia mass, however, such a softly damped movement will bring about great load variations at the wheel, causing wheel bounce under extreme conditions. It is at this point that the conflict with safety arises because the latter calls for the smallest possible dynamic wheel load variations.
In the interest of optimum comfort, a soft damper setting is chosen in straight-ahead driving over a road with only minor unevenness. On undulations, however, the ADS provides firm damping in order to prevent pitching. Upon receiving signals indicating an evasive manoeuvre or cornering, the system changes in time to a harder damper setting, even before lateral acceleration to any extent worth mentioning occurs, thus minimising wheel load variations. This illustrates the philosophy behind ADS: The damper setting provides for optimum ride comfort whenever comparatively little lateral stability is required, for instance in "normal" straight-ahead driving. As soon as lateral stability is called for, however (for cornering for instance), ADS changes over to firm damping - virtually in an anticipating manner in the case of steering manoeuvres so as to provide maximum safety reserves under all conditions. The damping is also adjusted to the appro- priate "hard" setting under sharp acceleration and braking and also when the car is heavily laden, in order to improve road-holding stability.
Five sensors are permanently monitoring the driving condition. They record vertical acceleration of wheel and bodywork, road speed, steering wheel angle and load. An electronic control unit processes the sensor signals, calculates the optimum damper setting at any given point in time, and activates the solenoid valves in the suspension's hydraulic system, the damper setting being modified by the opening or closing of these solenoid valves.
To ensure optimum comfort and maximum safety under all load conditions, ADS is coupled with automatic level adjustment and control. The latter ensures that the car remains at a constant level, at all four wheels and irrespective of the load and its distribution. At speeds above 120 km/h, the bodywork is automatically lowered by 15 mm to increase road-holding stability and reduce drag. To increase ground clearance, the driver can raise the car's level by 30 mm - however, only up to a speed of approx. 80 km/h. If this speed is exceeded, the car is automatically lowered to its normal level.
Reliable brakes
Sensitive brake response, balanced brake power distribution and generously dimensioned brakes are essential requirements for safe deceleration in every situation. At a very early stage in design, it emerged that compliance with Mercedes-Benz's own safety standards for brake reliability and brake power reserves in a car of this performance category would call for wheel brakes which could hardly be accommodated in conventional 15-inch wheels. For this reason, the new SLS are fitted with 16-inch wheels. All four wheels are decelerated by generously dimensioned disc brakes, the front brake discs being internally ventilated. The light-alloy rims, size 8Jx16, are fitted with 225/55 ZR 16 tyres. The anti-lock braking system (ABS) is a standard feature.Passive safety
Trend-setting technology
At Mercedes-Benz, vehicle occupant protection is traditionally a matter of top priority. This philosophy, which has been applied in the design and production of several generations of cars, has led to the development of many safety features which have created automotive history. These in- clude the anti-lock braking system (ABS) a feature developed and introduced into production by Mercedes-Benz - in addition to the airbag and the seat-belt tensioner. Mercedes-Benz is now also incorporating innovative safety technology into an open car, with features such as the raisable roll-over bar and the integral seat offering a high level of occupant protection.The raisable roll-over bar
Thanks to the raisable roll-over bar, a completely new fea- ture, the classical appearance of the open car has been re- tained. This bar also fulfils the stringent safety require- ments for occupant protection in overturning, which Mer- cedes-Benz also applies to its convertibles. When retracted,. the roll-over bar is located in front of the soft-top com- partment. It has two modes of operation: crash activation and comfort activation. In other words, it can also be ex- tended slowly if desired; the driver merely has to operate a switch, the actual work being performed by a spring-loaded hydraulic mechanism. In crash activation, the roll-over bar extends extremely rapidly in potentially dangerous situations without any action on the part of the driver. The spring components are then disengaged from the hydraulic system so that the roll-over bar can quickly reach its end position.Crash activation also occurs if the driver selects the com- fort mode in a hazardous situation. The bar can also be re- turned to its original position at the touch of a switch. This procedure is taken care of by the hydraulic system, whereby the spring mechanism is tensioned.
In crash activation, the roll-over bar extends within the space of about 0.3 seconds, with an average speed of about 1 m/s. Since the bar is fully padded with polyurethane, pos- sible contact with rear-seat passengers cannot cause injury.
Crash activation can only occur once a hazardous situation has been unmistakably established. To this end, a multi- functional control unit registers the vehicle's acceleration and assesses signals received from sensors measuring inclina- tion and rear-axle spring rebound; the control unit can thus recognise hazardous situations in which the vehicle could overturn. The catch is then released by means of an electro- magnetic actuator, allowing the roll-over bar to extend.
The roll-over bar consists of a U-shaped tube of high-quality steel, which when extended rests on the robustly dimensioned centre pillar.
The integral seat for increased safety
A new generation of seats is now being introduced in the SL: Mercedes-Benz has developed an integral seat which does not have much in common with its predecessors.This seat is based on a frame of pressure-diecast magnesium, which increases the transverse rigidity of the vehicle in a lateral collision.
The load-bearing frame has made it possible to incorporate the following functions into the fully electrically adjust- able seat, thus enhancing safety and operational comfort:
- three-point inertia- reel seat belt
- seat belt height adjustment
- head restraint adjustment
- seat-belt tensioner.
In this integrated occupant safety system, the seat-belt height adjustment is interconnected with the head restraint adjustment. Five electric motors move the seat squab and backrest into the most comfortable position, providing appro- priate seat-belt heights and head restraint positions for people of various heights.
These comfort features also serve to enhance occupant pro- tection. The belts fit optimally thanks to the individual sash guide height adjustment and the constant position of the lap strap.
As in all Mercedes-Benz passenger cars, the protective func- tion of the seat belt is enhanced by means of an electronic- ally activated seat-belt tensioner integrated into the seat, reducing belt slack.
A memory feature is available as an optional extra for stor- ing electronic adjustment settings of the seat squab, back- rest, head restraint positions and the seat-belt height.
The integration of the seat belt, including the inertia reel, into the seat itself places great significance on the secure locking of the backrest: only when this condition is ful- filled can the restraint system satisfactorily carry out its protective function. A highly refined, reliable, electronically monitored locking mechanism has now been developed. If the backrest is not properly engaged in position, the driver receives a visual and acoustical warning.
The airbag - an optional extra
In a frontal collision, the protection afforded by the seat belts can be further enhanced by the airbags for the driver and front-seat passenger. The latter takes the place of the glove compartment.To the end of 1988, the driver's airbag had been installed in a total of almost 500,000 vehicles, proving invaluable in the accidents investigated and analysed by Mercedes-Benz.
The electronic "crash sensor", incorporating microprocessor technology, controls the activation of both the seat-belt tensioners and the airbags. In the event of a frontal col- lision, this control unit assesses the severity of the acci- dent on the basis of the rate and duration of the vehicle's deceleration. If one or both of the programmed thresholds are exceeded, an electronics system transmits a command to ignite the gas generators. The seat-belt tensioners are ac- tivated as soon as the lower threshold is exceeded; if the upper threshold is also passed, the airbags for the driver and front-seat passenger also receive an activation command.
The airbag system is automatically tested every time the ig- nition is switched on and is continuously monitored when the car is in motion. The functional availability of the system is indicated by a control lamp.
It is a basic rule that the airbags can only fulfil their function adequately when the seat belts are being worn.
Optimum occupant protection
All Mercedes-Benz safety design features are aimed at pro- viding maximum protection for vehicle occupants. 75 per cent of tests carried out by Mercedes-Benz, serving to fulfil these safety requirements, are based on accidents actually occurring and go far beyond complying with legislation. The crash tests must fulfil the following criteria:- The passenger compartment must remain intact.
- The interior must incorporate large, energy-absorbing contact zones.
- The restraint systems must operate reliably.
- Umproblematical rescuing of the occupants must be assured.
- The fuel system must be leakproof.
In all design measures, the protection of the occupant com- partment is given absolute priority. Only when this basic requirement is met can safety features such as the restraint systems and padded contact areas completely fulfil their pro- tective function.
As accident analyses have shown, vehicles are subject to an extremely high degree of structural stress in the course of accidents actually occurring. This degree of stress is not taken into account by any legislation either in Europe or in the USA. This particularly applies to frontal collisions with oncoming traffic and with telegraph poles and to impacts to the rear, side and roof of the vehicle. For this reason, the following types of accident have been taken into account in the design of the SL body:
- frontal collision with 40% and 20% overlap.
- rear-end collision with 50% overlap.
- lateral impact, in some cases with extremely sharp and jagged movable barriers.
- asymmetrical roof impact with multiple overturning.
The preliminary evaluations and crash tests carried out prior to production have taken into account the structural peculi- arities of the open vehicle and have led to the following design measures to alleviate the consequences of frontal col- lisions:
- an occupant compartment which retains its longitudinal rigidity even in an off-set frontal collision.
- abosorption of some of the impact energy by the side of the front end not directly involved in the impact.
- spatial distribution of longitudinal forces from the front end to the occupant compartment via a specially devel- oped yoke.
- preventing the penetration of components through the fire wall, especially into the driver's footwell.
- a minimium of steering system displacement.
- low repair costs.
In spite of the car's relatively short rear-end overhang, an above-average level of occupant protection has also been achieved for rear-end collisions.
Especially in lateral impacts, the occupant compartment is subject to extreme stresses. By means of a series of meas- ures for increased transverse rigidity in the floor and seats, further improvements have been incorporated into the SL for this type of accident too. With additional reinforce- ment elements in the area of the tunnel, for example, the floor on the side of the vehicle away from the impact also absorbs some of the energy. In addition to this, the follow- ing structural features offer increased occupant protection:
- robustly dimensioned side members.
- seat rails on the side member designed to convey the impact forces directly to the stable seat squab frame.
- a dimensionally stable tunnel area with supported tunnel flanks.
- a high resistance to bending in the area of the connection points between the tunnel and the cross-member under the seat.
A significant feature is the considerably reduced door pene- tration brought about by the flush fit of the bottom of the doors with the side members and by the specially reinforced outside edge of the backrest frame, which is highly resistant to bending in lateral impacts.
In order to reduce the extent of injuries to pedestrians and cyclists, various alterations to the vehicles' shape and structure have been introduced in production, e.g.:
- a flat, strongly angled front surface with a rounded bon- net edge.
- an almost continuous transition to the bumper, with an energy-absorbing foam layer and a yielding outer skin.
- a flat bonnet extending almost to the windscreen.
- a windscreen wiper drive mechanism partly concealed by the bonnet and fitted with large-surface encapsulation.
- perforations in the wing supports under the edge of the bonnet for further yielding.
- partly recessed door handles.
A manifestation of aesthetics
The styling of the new Mercedes-Benz SL
The new Mercedes-Benz SL has its first public appearance. And self-assuredly it bears the modern features of a legend the beginnings of which date back 35 years. This SL image is above all the merit of the Mercedes-Benz designers, who today, after many years of creative work on the new Mercedes convertible, frankly admit: "This was the most difficult and most challenging task we ever faced".The team of chief designer Bruno Sacco has succeeded, however, not only to establish the link between the new SL with the internal designation of "R 129" and its distinctive predecessor model, successfully marketed since 1971, but also to consistently pursue the ideals of the legendary first 300 SL of the 50s.
The design of the new Mercedes-Benz SL is trailblazing just like all its predecessors, which have not lost any of their appeal and it is geared towards the future, anticipating the pace of time. Exotic fancies of shape, condemned to be short-lived, are not in line with this philosophy. Concessions in this respect would have meant to sacrifice the marquet identity and the SL image.
The uniqueness of an SL is emphasized by some distinctive design characteristics: the extended bonnet with the SL radiator grille - an aesthetically appealing entity achieved through the perfect symbiosis of shape and function the squat roof as well as the compact rear section. On the basis of these design features, the newcomer, too, will occupy its place in the row of the "typical SL models".
There is hardly a greater compliment for the Mercedes-Benz designers. For "typical SL" also stands for classic elegance, for dynamics and power. The reasons for the fact that the new SL suggests such concepts as tradition, high value, prestige, safety, comfort and performance by its mere outer shape are to be found in a complex and subtle design process.
The greatest difficulty the Mercedes-Benz designer faced besides creating a typical SL concept with stylistic attractiveness reaching beyond the turn of the millenium, was that they had to reconcile the two appearances of the new SL - with the soft top fully retracted and with the roof (soft or hard top) in position. This aim of the total stylistic harmonization of all appearances has been achieved. This has not least been possible due to the development of numerous innovative design features such as the sensor-controlled roll-over bar and an integral belt and seat design.
All these examples show that Mercedes-Benz styling is more than a mere framework for perfect engineering. Mercedes-Benz styling forms a perfect unity with technology. All the more precious appears the dynamic, powerful yet not forced flow of lines of the new SL series. This is achieved by the following design measures:
- extended bonnet reaching low down to the front short and compact-shaped roof
- rear with compact yet flowing lines
- even-surfaced sides with an aerodynamically streamlined wedge shape roof construction forming an aesthetic entity with the bodywork
- organically embedded windscreen
- inclined A-pillar gently moving up from the front fender with new dirt-deflecting system sportily shaped flared wheel arches
- refined appealingly shaped and newly dimensioned SL radiator grille with anodized aluminium louvres in titanium anthracite in shape and function logically integrated head and rear lamps
The exterior design is complemented by a generous interior, adding to the powerful appeal of the bodywork, which is characterized by:
- a safety standard unique for a convertible
- great stability
- hitherto unattained functionality and ergonomy
- extraordinary comfort standards
- a sound dose of sportiness
All these individual measures together with an appealing overall shape and top-class engineering come to form a perfect entity: the new Mercedes-Benz SL. This new SL furnishes further proof of the leading role Mercedes-Benz design plays in the automotive world. There are certain styling principles underlying the work of the Mercedes-Benz designers. Some of them are:
- With the increasing similarity of technical and stylistic features, the symbol value and character of a car are characteristics gaining increasing importance. Mercedes cars emit signals which are unique in their classic appeal.
- There is a permanent solution neither to style nor to technology. This is why for the Mercedes-Benz designers automobile styling is a continuous innovative process aiming at increasing the aesthetic experience enjoyed with Mercedes-Benz products.
- Mercedes-Benz design is a reflection of Mercedes-Benz quality. Technical functions and traditional values meet in an aesthetic shape to form a convincing symbiosis.
It was only in this way that the new Mercedes-Benz SL was able to become not the stylistic fashionable end in itself but the characterful new SL. Powerful in its appearance yet with amazingly compact dimensions. Although the new-generation SL opens up new design dimensions, it remains true to the design philosophy characteristic of Mercedes-Benz: it is as close to the SL introduced in 1971 (vertical product homogeneity) as to all contemporary Mercedes-Benz models (horizontal product homogeneity).